1. Field of the Invention
The present invention relates to a solder resist printing process in the fabrication of a printed circuit board (PCB), and more particularly to a technique for forming a solder resist pattern by laminating a thermosetting film on both sides of a substrate and irradiating a laser beam to the upper surface of the thermosetting film according to a solder mask to selectively remove the thermosetting film.
2. Description of the Related Art
The term ‘resist’ used herein refers to a film that protects the underlying substrate from a certain treatment or reaction in the fabrication of PCBs. The term ‘etching resist’ used herein means a film that protects the underlying substrate against etching, and acts to protect conducting parts from a chemical attack. Likewise, the term ‘plating resist’ means a film that protects a specific area from plating.
The term ‘solder’ used herein refers to braze. Accordingly, the term ‘solder resist’ refers to a film that covers a wiring pattern to prevent undesired connections from forming between electrical components due to a brazing during mounting the electrical components on a substrate. The solder resist plays a roll in not only the protection of circuits formed on the surface of a PCB substrate but also the insulation between the circuits, and is generally used in the form of paint.
Since the wiring pattern is formed by etching a copper foil clad on a substrate, it is like bare wires having no insulating cover. As PCBs are highly densified, the spacing between wirings is reduced. This narrow spacing causes problems of short-circuiting and cross connection between adjacent wirings, like electric wires without insulating covers. In particular, when electric parts are mounted on a PCB, they are exposed to molten dust from the PCB surface and thus undesirable solder bridges may occur. These solder bridges lead to important defects which impede normal operation of electronic devices.
In order to prevent these defects, it is necessary to coat the bare wires. The solder resist masks regions other than regions around lands (areas where electric components are mounted), where the electric components are brazed. The solder resist is also referred to as a so-called ‘solder mask’ in view of its masking function. The process for applying the solder resist on predetermined regions is generally called a ‘printing process’.
FIG. 1 shows a basic configuration of a common PCB on which a solder resist is applied. Circuit patterns formed on PCBs are typically very complex. The circuit pattern of FIG. 1 is briefly shown for the purpose of illustration.
After a printed circuit board is fabricated through various processes, regions 12 other than circuit patterns 11 are coated with a solder resist in order to prevent the opening and short-circuiting of the circuit patterns.
A photosensitive resist ink for a solder resist is called a “photo solder resist”, abbreviated PSR. The PSR is predominantly green, and is made of a resin having a high resistance even at brazing temperature.
The PSR is cured by ultraviolet (UV) light and heat.
Conventional processes for solder resist printing predetermined regions on a substrate with a circuit pattern formed thereon are accompanied by burial of via holes, application of a PSR, precuring of the PSR and final curing of the PSR, in this order.
Via holes of a PCB into which no components are inserted are buried with the PSR to prevent the oxidation of inner walls of the PCB and to facilitate the handling of the PCB during insertion of components into the PCB. The burial step is carried out by a screen printing process using a screen with a pattern printed thereon. At this time, the printed pattern varies according to the sizes and positions of via holes. The substrate with completely buried via holes is dried in a vertical position in order to prevent ink from flowing out of the via holes. In addition to the burial of via holes by the screen printing process, a photolithography process is available for burying the via holes.
After burying the via holes, the PSR is applied on the entire surface of the substrate. Then, a primary drying process is carried out so as not to damage an ink layer formed during applying the PSR. The application of the PSR is carried out by a process including screen printing, roller coating, curtain coating, spray coating, etc.
The screen printing is a process in which a solder resist pattern is directly printed using a screen, and the solder resist pattern is formed through light-exposure and development.
According to the roller coating process, a PSR is thinly spread over a rubber roller and then coated onto a substrate. At this time, the PSR has a viscosity lower than that used in the screen printing process. The roller coating process has drawbacks that the thickness of a resist to be coated cannot be adjusted depending on the type of substrate used and uniform coating is not easily achieved. In addition, the roller coating process has an optical anisotropy problem.
The optical anisotropy problem occurs when both sides of a semi-transparent substrate are exposed to light. That is, when a light penetrates the substrate, cloudy images are formed on the opposite side. ‘Back light exposure’ is used as an alternative expression for optical anisotropy.
In the curtain coating process, a PSR having a viscosity lower than that used in the roller coating process is used. According to the curtain coating method, the PSR is discharged through a slit to form a membrane in the form of a curtain, through which a substrate is coated.
The curtain coating process has advantages that very uniform coating quality can be obtained and various sized substrates can be used. However, the curtain coating process has problems of high cost and optical anisotropy.
The spray coating is a process in which a resist ink is sprayed to coat a substrate. According to the spray coating process, the thickness of the coated layer is easy to control. However, the spray coating process has problems of high cost and optical anisotropy.
After applying the PSR, a pattern is formed by closely adhering an artwork film with a solder resist pattern printed thereon to the substrate, exposing to light and developing.
Thereafter, a resin contained in the PSR is cured. When the curing is insufficient, there may be a defect that a PSR resist film is split in the following processes. As mentioned above, the PSR may be thermal curable, UV-curable, thermal plus UV curable composite types, or the like.
FIG. 2 shows an embodiment of a conventional solder resist printing process. Specifically, FIG. 2 is a flowchart showing a process for printing a solder resist of a substrate for an FC-BGA (Flip Chip Ball Grid Array) type package. Referring to FIG. 2, the substrate is a multilayer printed circuit board, and a UV-curable PSR is used as the solder resist.
In step S201, a scrubbing process is carried out on both sides of the substrate to improve the adhesion between the PSR and the substrate.
In step S202, the PSR is applied to wet the substrate. The application of the photo solder resist is carried out by a process including screen printing, roller coating, curtain coating, spray coating, etc.
A first predrying step is carried out (step S203), a second printing step of the PSR is carried out (step 204), and a second predrying step is carried out (step S205). In step S206, a PET resin can be laminated to improve light exposure. Step S206 can be omitted. The substrate is exposed to UV light to cure a film (step S207), and then developed (step S208).
In step S209, a post-exposure step is carried out to promote a crosslinking reaction in the film. In step S210, a final drying is carried out to remove solvents, etc., and finally the printing process of the solder resist is completed.
As discussed above, conventional solder resist printing processes require a number of steps.
In addition, the conventional solder resist printing processes have a problem that inflow of foreign materials is likely to happen during the printing. The inflow of foreign materials originates from the used wet ink.
In a conventional process using a photosensitive solder resist ink, foreign materials may enter during light exposure to remove a solder resist on desired regions parts.
Furthermore, unsuitable drying conditions in the drying steps can cause the non-uniform colors of a solder resist to be printed.